Method/apparatus for NMR imaging using an imaging scheme sensitive to inhomogeneity and a scheme insensitive to inhomogeneity in a single imaging step

ABSTRACT

A nuclear magnetic resonance imaging scheme for imaging living body information related to the physiological function change in the living body. In this scheme, the pulse sequence realizes a first imaging scheme sensitive to functional information of the body to be examined and a second imaging scheme insensitive to the functional information of the body to be examined so as to obtain first and second types of the nuclear magnetic resonance images corresponding to the first and second imaging schemes, respectively, by a single execution of this pulse sequence. The functional information of the body to be examined is then obtained by processing the first and second types of the nuclear magnetic resonance images.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nuclear magnetic resonance imaging,and more particularly, to a nuclear magnetic resonance imaging suitablefor imaging physiological function information of the interior of thebody to be examined at high precision.

2. Description of the Background Art

In recent years, many medical diagnostic systems using the nuclearmagnetic resonance imaging (MRI) apparatus have been developed.

As well known, the nuclear magnetic resonance imaging is a method forimaging microscopic chemical and physical information of matters byutilizing the nuclear magnetic resonance phenomenon in which the energyof a radio frequency magnetic field rotating at a specific frequency canbe resonantly absorbed by a group of nuclear spins having uniquemagnetic moments which are placed in a homogeneous static magneticfield.

In this nuclear magnetic resonance imaging, the images can be obtainedin various contrasts such as the image in contrast emphasizing thelongitudinal relaxation time T₁ of the nuclear spins (T₁ image), theimage in contrast emphasizing the transverse relaxation time T₂ of thenuclear spins (T₂ image), the image in contrast emphasizing the densitydistribution of the nuclear spins (density image), and the image incontrast emphasizing the parameter T₂ * which reflects both thetransverse relaxation time T₂ and the sudden phase change of the nuclearspins due to the microscopic magnetic field inhomogeneity within avoxel.

On the other hand, as described in S. Ogawa et al.:"Oxygenation-Sensitive Contrast in Magnetic Resonance Image of RodentBrain at High Magnetic Fields", Magnetic Resonance in Medicine 14, pp.68-78, 1990, it is known that, among the hemoglobin contained in bloodof the living body, the oxyhemoglobin contained in abundance in thearterial blood is diamagnetic, while the deoxyhemoglobin mainlycontained in the venous blood is paramagnetic. Then, as described in R.M. Weisskoff et al.: "MRI Susceptometry: Image-Based Measurement ofAbsolute Susceptibility of MR contrast Agents and Human Blood", MagneticResonance in Medicine 24, pp. 375-383, 1992, it is also known that thediamagnetic oxyhemoglobin does not disturb the local magnetic field verymuch (magnetic susceptibility difference of 0.02 ppm with respect to theliving body tissues), but the paramagnetic deoxyhemoglobin hassufficiently large magnetic susceptibility difference with respect tothe surrounding tissues (magnetic susceptibility difference of 0.15 ppmwith respect to the living body tissues) to disturb the magnetic fieldso that the parameter T₂ * is going to be shortened.

Also, as described in J. A. Detre, et al.: "Perfusion Imaging", MagneticResonance in Medicine 23, pp. 37-45, 1992, in some imaging schemes ofthe nuclear magnetic resonance imaging, when the amount or the speed ofthe local blood flow within the living body tissues, the relaxation time(such as T₁) of the living body seemingly appears to have changed, andthe image contrast can be changed.

By utilizing the above noted properties, it is possible to image thechange of the blood flow or the change of the oxygen density in blooddue to the physiological function such as the cell activity within theliving body tissues including the activation of the visual area in thebrain cortex caused by the light stimulation, as described for examplein K. K. Kwong et al.: "Dynamic magnetic resonance imaging of humanbrain activity during primary sensory stimulation", Proc. Natl. Acad.Sci. USA, Vol. 89. pp. 5675-5679, June 1992. Conventionally, the imagingscheme used in this type of imaging has been the echo planar schemeusing the pulse sequence as shown in FIG. 1 or the gradient echo schemeusing the pulse sequence as shown in FIG. 2.

However, in these imaging schemes, the signal change (image contrastchange) caused by the physiological function within the living body isquite minute. For this reason, conventionally, this minute signal changehas been detected by calculating the difference or the correlation ofthe images before and after the occurrence of the physiological functionphenomenon, as described in R. T. Constable, et al.: "Functional BrainImagings at 1.5 T using Conventional Gradient Echo MR ImagingTechniques", Magnetic Resonance Imaging, Vol. 11, pp. 451-459, 1993. Inaddition, there has been an attempt to comprehend the physiologicalfunction quantitatively by calculating the change of the blood flowamount or the oxygen density in blood from the change of the contrastintensity or the phase in the images.

However, in such a conventional method, when the position displacementdue to the body movement between two images occurs, it becomesimpossible to detect such a minute change accurately. In fact, it iswell known that the position and the size of the brain can change insynchronization with the heart beat, as described in B. P. Poncelet, etal.: "Brain Parenchyma Motion: Measurement with Cine Echo-Planar MRimaging", Radiology, Vol. 185, pp. 645-651, December 1992. Thus, in theconventional method, because of the influence of the body movement dueto the breathing or the heart beat, it has been impossible to accuratelydetect the signal change (image contrast change) caused by thephysiological function such as the cell activity in the living body.

On the other hand, it is also well known that the image distortion canbe caused in the nuclear magnetic resonance imaging when the staticmagnetic field distribution is inhomogeneous, and this image distortionbecomes particularly noticeable in the imaging scheme for the T₂ * imagewhich is used in detecting the physiological function phenomenon such asthe cell activity in the living body. However, when such an imagedistortion is present, it is impossible to accurately detect theposition of the physiological function change such as the cell activityin the living body.

Moreover, in a case of calculating an average image from a plurality ofimages obtained by the repeated imaging operations in order to improvethe signal to noise ratio of the image, or carrying out the processingamong a plurality of images in order to detect the physiologicalfunction phenomenon such as the cell activity in the living body, whenthe signal strength or the imaged portion changes depending on theimaging conditions or the system states, it is impossible to accuratelydetect the physiological function change such as the cell activity inthe living body by the processing among the images.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand an apparatus for nuclear magnetic resonance imaging, capable ofimaging living body information such as the blood flow change or theoxygen density in blood related to the physiological function change inthe living body.

According to one aspect of the present invention there is provided amethod of nuclear magnetic resonance imaging, comprising the steps of:imaging a body to be examined placed in a homogeneous static magneticfield by applying radio frequency magnetic field and gradient magneticfields according to a pulse sequence, detecting nuclear magneticresonance signals emitted from the body to be examined in response tothe radio frequency magnetic field and the gradient magnetic fields, andprocessing the nuclear magnetic resonance signals to construct nuclearmagnetic resonance images; controlling the pulse sequence to realize afirst imaging scheme sensitive to functional information of the body tobe examined and a second imaging scheme insensitive to the functionalinformation of the body to be examined so as to obtain first and secondtypes of the nuclear magnetic resonance images corresponding to thefirst and second imaging schemes, respectively, by a single execution ofthe imaging step; and obtaining the functional information of the bodyto be examined by processing the first and second types of the nuclearmagnetic resonance images.

According to another aspect of the present invention there is providedan apparatus for nuclear magnetic resonance imaging, comprising: imagingmeans for imaging a body to be examined placed in a homogeneous staticmagnetic field by applying radio frequency magnetic field and gradientmagnetic fields according to a pulse sequence, detecting nuclearmagnetic resonance signals emitted from the body to be examined inresponse to the radio frequency magnetic field and the gradient magneticfields, and processing the nuclear magnetic resonance signals toconstruct nuclear magnetic resonance images; control means forcontrolling the pulse sequence to realize a first imaging schemesensitive to functional information of the body to be examined and asecond imaging scheme insensitive to the functional information of thebody to be examined so as to obtain first and second types of thenuclear magnetic resonance images corresponding to the first and secondimaging schemes, respectively, by a single execution of the pulsesequence; and processing means for obtaining the functional informationof the body to be examined by processing the first and second types ofthe nuclear magnetic resonance images.

Other features and advantages of the present invention will becomeapparent from the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a pulse sequence for the conventional echo planarscheme.

FIG. 2 is a diagram of a pulse sequence for the conventional gradientecho scheme.

FIG. 3 is a block diagram of a nuclear magnetic resonance imagingapparatus suitable for the present invention.

FIG. 4 is a diagram of a pulse sequence for the first embodiment of theimaging scheme for imaging the physiological function informationaccording to the present invention.

FIG. 5 is a flow chart for the operation to detect the changed portionby using the imaging scheme of FIG. 4 with and without stimulation.

FIG. 6 is a flow chart for the static magnetic field inhomogeneitycorrection processing used in the operation of FIG. 5.

FIG. 7 is a flow chart for the geometrical correction processing used inthe operation of FIG. 5.

FIG. 8 is a flow chart for the signal strength correction processingused in the operation of FIG. 5.

FIG. 9 is a diagram of a pulse sequence for the second embodiment of theimaging scheme for imaging the physiological function informationaccording to the present invention.

FIG. 10 is a flow chart for the operation to quantitatively detect thechanged portion by using the imaging scheme of FIG. 9 with and withoutstimulation.

FIG. 11 is a flow chart for the T₂ normalization processing used in theoperation of FIG. 10.

FIG. 12 is a flow chart for the operation to quantitatively detect thechanged portion by using the imaging scheme of FIG. 9 with and withoutstimulation and the phase correction processing.

FIG. 13 is a diagram of a pulse sequence for the third embodiment of theimaging scheme for imaging the physiological function informationaccording to the present invention.

FIG. 14 is a diagram of a pulse sequence for the fourth embodiment ofthe imaging scheme for imaging the physiological function informationaccording to the present invention.

FIG. 15 is a diagram of a pulse sequence for the fifth embodiment of theimaging scheme for imaging the physiological function informationaccording to the present invention.

FIG. 16 is a diagram of a pulse sequence for the sixth embodiment of theimaging scheme for imaging the physiological function informationaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the various embodiments of a method and an apparatus for nuclearmagnetic resonance imaging according to the present invention will bedescribed in detail.

First, the MRI apparatus suitable for the present invention has aconfiguration as shown in FIG. 3, which includes a main magnet 1 forgenerating the static magnetic field, shim coils 8 for adjusting thehomogeneity of the static magnetic field, and gradient coils 5 forgenerating gradient magnetic fields, which are driven by a main magnetpower source 2, a shim coil power source 4, and a gradient coil powersource 6, respectively, such that the homogeneous static magnetic fieldand the gradient magnetic fields having linear gradient fielddistribution in three orthogonal directions can be applied onto a bodyto be examined 7.

In addition, there is provided a probe 9 to which radio frequencysignals are transmitted from a transmitter 10 such that radio frequencymagnetic fields can be applied from this probe 9 to the body to beexamined 7. This probe 9 is also used for receiving the nuclear magneticresonance signals emitted from the body to be examined 7 in response tothe application of these magnetic fields, but a separate signal detectormay be provided in addition to this probe 9 if desired.

The nuclear magnetic resonance signals received by the probe 9 aredetected by the quadrature detection at a receiver 11, and A/D convertedat data acquisition unit 13, and then supplied to a computer 14.

Here, the operations of the main magnet power source 2, the shim coilpower source 4, the gradient coil power source 6, the transmitter 10,the receiver 11, and the data acquisition unit 13 are controlled by asystem controller 12, which in turn is controlled from a console 15through the computer 14.

At the computer 14, the image reconstruction processing is carried outaccording to the nuclear magnetic resonance signal data supplied fromthe data acquisition unit 13, so as to obtain the image data. Theobtained images are then displayed on a display 16. This computer 14 aswell as a bed 8 for mounting the body to be examined 7 thereon arecontrolled from the console 15.

In this configuration of FIG. 8, the pulse sequence for acquiring theimage data within slice planes in the body to be examined 7 as well as astimulation device 17 for providing a stimulation such as lights orsounds to the body to be examined 7 are controlled by the systemcontroller 12.

In the following, the imaging schemes for imaging the physiologicalfunction information of the interior of the body to be examined by usingthe MRI apparatus of FIG. 3 will be described. Here, the exemplary caseof using the parameter T₂ * as a functional information and theparameter T₂ as a shape information will be explained as an illustrativeexample.

Now, FIG. 4 shows the pulse sequence of the first embodiment of theimaging scheme for imaging the physiological function information in thebody to be examined. In this FIG. 4, RF indicates the radio frequencymagnetic fields (RF pulses), while Cs, Gr, and Ge indicate the slicing,reading, and phase encoding gradient magnetic Fields, respectively, andSig indicates the nuclear magnetic resonance signals (NMR signals).Here, the slicing gradient magnetic field Gs is provided for exciting adesired slice region in the body to be examined 7, the reading gradientmagnetic field Gr is for provided for reading out the nuclear magneticresonance signals, and the phase encoding gradient magnetic field Ge isprovided for encoding position information into phases of the nuclearmagnetic resonance signals.

In this first embodiment, the 90° RF pulse and the slicing gradientmagnetic field Gs are applied to excite the desired slice region togenerate the free induction decay (FID) NMR signals first. Then, thefirst data acquisition is carried out by the echo planar scheme in whichthe reading gradient magnetic field Gr is repeatedly switched topositive and negative alternately to generate a plurality of echosignals, while the phase encoding gradient magnetic field Ge is appliedat a timing of each echo signal. Then, the 180° RF pulse is applied togenerate the spin echo signals, and the second data acquisition iscarried out by the similar echo planar scheme.

Then, after the appropriate preliminary processing, the data obtained bythe first and second data acquisitions are complex Fourier transformedto produce two images. At this point, the time intervals from the centerof the 90° RF pulse to the data at an origin of two dimensionallyarranged data for the first and second data acquisitions are denoted asATE and TE, respectively. In these two images, the image obtained fromthe first data acquisition is going to be the T₂ * image reflecting thetransverse relaxation time T₂ of the nuclear spins and the microscopicmagnetic field inhomogeneity within a voxel, while the image obtainedfrom the second data acquisition is going to be the T₂ image reflectingthe transverse relaxation time T₂ of the nuclear spins. The actual timeintervals ΔTE and TE for imaging the head portion are typically 40 to 60msec for ΔTE, and 100 to 120 msec for TE.

Here, in the first half of the imaging sequence of FIG. 4, the NMRsignals are detected in a state in which the phases of the nuclear spinsare disturbed by the microscopic magnetic field inhomogeneity, so thatthe T₂ * image obtained by the first data acquisition is the image whichis easily affected by the microscopic magnetic field inhomogeneity andtherefore it can serve as the image sensitive to the functionalinformation of the body to be examined. On the other hand, in the secondhalf of the imaging sequence of FIG. 4, the NMR signals are detected ineither a state before the phases of the nuclear spins are disturbed, ora state in which the phases of the nuclear spins are re-aligned, so thatthe T₂ image obtained by the second data acquisition is the image whichis hardly affected by the microscopic magnetic field inhomogeneity andtherefore it can serve as the image sensitive to the shape informationof the body to be examined.

Now, by using these T₂ and T₂ * images obtained by the single continuousimaging sequence of FIG. 4, the image of the physiological functioninformation can be obtained as follows. Namely, when the phasedistributions in these T₂ and T₂ * images are I₂ r(x, y) and I₂ *r(x,y), the phase difference image φ(x, y) is given by the followingequation (1).

    Φ(x,y)=tan .sup.-1 (IM[I.sub.2 r(x,y)]/RE[I.sub.2 r(x,y)])-tan .sup.-1 (IM[I.sub.2 *r(x,y)]/RE[I.sub.2 *r(x,y)])                 (1)

Then, from this phase difference image Φ(x, y), the static magneticfield distribution ΔH(x, y) can be calculated by the following equation(2).

    ΔH(x,y)=Φ(x,y)/(γ·ΔTE)      (2)

where γ is a gyromagnetic ratio of the target nuclear spins.

This static magnetic field distribution ΔH(x, y) provides the imageindicative of the microscopic magnetic field inhomogeneity within theimaged slice region, and this can reflect the local magnetic fielddisturbance caused by the paramagnetic deoxyhemoglobin in blood forexample. Consequently, by executing the above imaging sequence to takethe T₂ and T₂ * images together in at least one of the states before,while, or after the body to be examined is stimulated by a stimulationwhich causes the physiological function change, it becomes possible toobtain the above static magnetic field distribution reflecting the localmagnetic field disturbances caused by the physical property of theinterior of the body to be examined such as the oxyhemoglobin content inblood.

Furthermore, by executing the above imaging sequence to take the T₂ andT₂ * images together in at least two of the states before, while, orafter the body to be examined is stimulated by a stimulation whichcauses the physiological Function change, it becomes possible to derivethe change in the static magnetic Field distributions obtained from twostates, and this can provide the living body information such as thechange of the blood flow or the change of the oxygen density in blooddue to the change of the physiological function such as the cellactivity within the living body tissues.

Therefore, this first embodiment can provide a scheme For nuclearmagnetic resonance imaging of living body information such as the bloodflow change or the oxygen density in blood related to the physiologicalfunction change in the living body.

Now, using the above imaging sequence of FIG. 4, it is possible to takeimages of the head portion in two states of with and without stimulationsuch as the light or sound stimulation given by the stimulation device17, for example. In such a case, in the T₂ * images obtained by thefirst data acquisition with and without stimulation, the change occursin the oxygen density in blood or the local blood flow due to theactivation of the particular portion of the brain cells in reaction tothe stimulation, so that the T₂ * contrast changes in accordance to thechange of the magnetic susceptibility of the local tissues in a vicinityof the activated portion. On the other hand, in the T₂ images obtainedby the second data acquisition with and without stimulation, thetransverse relaxation time T₂ of the tissues remains unchangedregardless of the presence or absence of the stimulation, so that thesame T₂ contrast is obtained in both of the T₂ images with and withoutstimulation.

Then, the portion in which the brain cells have been activated by thestimulation can be detected by the operation according to the flow chartof FIG. 5 as follows.

Namely, from the acquired image data at the initial step 50, thecorrection processing concerning the static magnetic field inhomogeneity(step 51), the geometrical correction processing concerning the bodymovement of the living body (step 52), and the correction processingconcerning the signal strength among images (step 53) are sequentiallyand repeatedly carried out until the desired processed images areobtained (step 54). Then, after the desired processed images areobtained, the changed portion in which the brain cells have beenactivated by the stimulation is detected from the change in theprocessed T₂ * images with and without stimulation. Finally, theinformation on the detected portion with the activated brain cells canbe presented in a form of the image display (step 56).

In further detail, the static magnetic field inhomogeneity correctionprocessing at the step 52 of FIG. 5 can be carried out according to theflow chart of FIG. 6. Namely, from the phase distributions of the T₂ andT₂ * images obtained without stimulation at steps 61 and 62, the staticmagnetic field distribution ΔH can be calculated by using the equations(1) and (2) described above (step 63), and then the correction of theimage distortion caused by the static magnetic field inhomogeneity canbe carried out according to the calculated static magnetic fielddistribution, by using the known correction method such as thatdisclosed in Japanese Patent Application Laid Open No. 64-56042.

Next, the geometrical correction processing at the step 52 off FIG. 5can be carried out according to the flow chart off FIG. 7. Namely, fromthe T₂ images obtained with and without stimulation at steps 71 and 72,the coordinate transformation formula for transforming the coordinatesin these T₂ images one another can be determined (step 73), and then thegeometrical correction of the images according to the determinedcoordinate transformation formula can be carried out (step 74). Here,the coordinate transformation formula can be given by the affinetransformation defined by the following equation (3). ##EQU1##

In this affine transformation of the equation (3), the coefficients a,b, c, d, e, and f can be determined by using the T₂ images obtained withand without stimulation or the processed T₂ images resulting from theabove described static magnetic field inhomogeneity correctionprocessing which indicate the presence and absence of the stimulation.Namely, when the pixel values in these T₂ images with and withoutstimulation are I₂ sc(x, y) and I₂ rc(x, y), the above coefficients aredetermined such that the value defined by the following expression (4)becomes minimum. ##EQU2## where M and N are numbers of image pixels in xand y directions.

For this geometrical correction processing, it is also possible to usethe other known methods such as those using the Helmert transformationor the quasi-affine transformation, or the method in which the contourextraction is carried out from the T₂ images with and withoutstimulation, and the coordinate transformation Formula is derived fromthe pattern matching of the extracted contours.

Next, the signal strength correction processing at the step 53 off FIG.5 can be carried out according to the flow chart of FIG. 8. Namely, fromthe T₂ images obtained with and without the stimulation at steps 81 and82, the signal strength correction values are calculated (step 83), andthe signal strength correction using the calculated signal strengthcorrection values can be carried out (step 84).

Here, as the method for calculating the signal strength correctionvalues, the signal correction magnification rate for the signal strengthat each picture element or over an entire image can be obtained from theT₂ images with and without stimulation or the processed T₂ images.

It is to be noted that all or only a part of the above describedcorrection processings of the steps 51, 52, and 53 of FIG. 5 can becarried out according to the need, and their orders may be changed ifdesired.

Next, the judgement as to whether the desired processed images areobtained or not at the step 54 of FIG. 5 can be made as follows. Namely,when the pixel values in the processed T₂ images with and withoutstimulation are I₂ sc(x, y) and I₂ rc(x, y), the judgement can be madeby determining whether the value defined by the following expression (5)becomes less than or equal to the desired value. ##EQU3## where M and Nare numbers of image pixels in x and y directions, and A is the signalcorrection magnification rate which is either a constant or a functionof x and y.

Next, the detection of the changed portion at the step 55 of FIG. 5 canbe carried out as follows. Namely, when the pixel values in theprocessed T₂ * images with and without stimulation are I₂ sc(x, y) andI₂ rc(x, y), the changed portion can be detected by the thresholding thedifference image defined by the following equation (6).

    Idif(x,y)=I.sub.2 ·sc(x,y)-I.sub.2 ·rc(x,y)(6)

For this detection of the changed portion, it is also possible the usethe statistical data processing methods such as the X test method.

Next, FIG. 9 shows the pulse sequence of the second embodiment of theimaging scheme for imaging the physiological function information in thebody to be examined.

In this second embodiment, the 90° RF pulse and the slicing gradientmagnetic field Gs are applied to excite the desired slice region togenerate the free induction decay (FID) NMR signals first. Then, thefirst data acquisition is carried out by the echo planar scheme in whichthe reading gradient magnetic field Gr is repeatedly switched topositive and negative alternately to generate a plurality of echosignals, while the phase encoding gradient magnetic field Ge is appliedat a timing of each echo signal. Then, the 180° RF pulse is applied togenerate the spin echo signals, and the second data acquisition iscarried out by the similar echo planar scheme. Thereafter, the similarapplication of the 180° RF pulse and the gradient magnetic fields arerepeated as many times as necessary (four times in FIG. 9) to make thefurther data acquisitions.

Then, after the appropriate preliminary processing, the data obtained bythe data acquisitions are complex Fourier transformed to produce aplurality of images. In these images, the image obtained from the firstdata acquisition is going to be the T₂ * image reflecting the transverserelaxation time T₂ of the nuclear spins and the microscopic magneticField inhomogeneity within a voxel as in the first embodiment, while theimages obtained from the second and subsequent data acquisitions aregoing to be the T₂ images reflecting the transverse relaxation time T₂of the nuclear spins. Here, however, the echo time (that is, the timeinterval from the center of the 90° RF pulse to the data at an origin oftwo dimensionally arranged data For each data acquisition) is differentfor different data acquisition, so that the images obtained from thesecond and subsequent data acquisitions are going to be in contrastemphasizing the different T₂ values.

Then, similarly as in the first embodiment described above, using theabove imaging sequence of FIG. 9, it is possible to take images of thehead portion in two states of with and without stimulation such as thelight or sound stimulation given by the stimulation device 17, forexample. In such a case, in the T₂ * images obtained by the first dataacquisition with and without stimulation, the change occurs in theoxygen density in blood or the local blood flow due to the activation ofthe particular portion of the brain cells in reaction to thestimulation, so that the T₂ * contrast changes in accordance to thechange of the magnetic susceptibility of the local tissues in a vicinityof the activated portion. On the other hand, in the T₂ images obtainedby the second and subsequent data acquisitions with and withoutstimulation, the transverse relaxation time T₂ of the tissues remainsunchanged regardless of the presence or absence of the stimulation, sothat the same T₂ contrast is obtained in both of the T₂ images with andwithout stimulation.

Then, the portion in which the brain cells have been activated by thestimulation can be detected quantitatively by the operation according tothe flow chart of FIG. 10 as follows.

Namely, as in the case of FIG. 5 for the first embodiment describedabove, from the acquired image data at the initial step 50, thecorrection processing concerning the static magnetic field inhomogeneity(step 51), the geometrical correction processing concerning the bodymovement of the living body (step 52), and the correction processingconcerning the signal strength among images (step 53) are sequentiallyand repeatedly carried out until the desired processed images areobtained (step 54). Then, after the desired processed images areobtained, the the T₂ normalization processing for calculating the T₂value distribution and normalizing the signal strength according to thecalculated T₂ value distribution is carried out (step 101) in order toenable the quantitative detection of the changed portion.

This T₂ normalization processing at the step 101 of FIG. 9 can becarried out according to the flow chart of FIG. 11. Namely, from aplurality of T₂ images obtained by the second and subsequent dataacquisitions at the steps 110-1 to 110-N, the T₂ value at each pictureelement is calculated according to the change of the signal strength ofthe corresponding image data among these T₂ images to obtain the T₂value distribution (step 111). Then, according to this obtained T₂ valuedistribution, the normalization of the signal strength in the T₂ *images obtained by the first data acquisition is carried out (step 112).

Next, the changed portion in which the brain cells have been activatedby the stimulation is quantified by calculating the change of the signalstrength in the processed T₂ * images with and without stimulation (step102). In this manner, it becomes possible to quantitatively detects thechange of the local blood flow or the oxygen density in blood due to theactivation of the brain cells as the change of the magneticsusceptibility of the local tissues, regardless of the T₂ value of thetissues.

Similarly, instead of quantifying the function information from thechange of the signal strength as in FIG. 10, it is also possible toquantify the function information from the change of the phaseinformation in the images as shown in FIG. 12. In this case, instead ofthe T₂ normalization processing at the step 101 in FIG. 10, the phasecorrection processing concerning the phase error due to the staticmagnetic field inhomogeneity must be carried out (step 121) before thequantification of the changed portion at the step 102. Here, however,this phase correction processing can be carried out by utilizing thestatic magnetic field distribution information obtained in the staticmagnetic field inhomogeneity correction processing described above.

Next, FIG. 13 shows the pulse sequence of the third embodiment of theimaging scheme for imaging the physiological function information in thebody to be examined.

In this third embodiment, instead of the echo planar scheme used in thefirst and second embodiments described above, the high speed spin echoscheme for generating a plurality of echo signals by the repeatedapplication of the 180° RF pulses is used. Thus, in this case, the 90°RF pulse and the slicing gradient magnetic field Gs are applied toexcite the desired slice region to generate the free induction decay(FID) NMR signals first. Then, the 180° RF pulses and the slicinggradient magnetic field Gs are repeatedly applied while between thesuccessive applications of the 180° RF pulse and the slicing gradientmagnetic field Gs, the reading gradient magnetic field Gr is repeatedlyswitched to positive and negative alternately to generate a plurality ofecho signals, and the phase encoding gradient magnetic Field Ge isapplied before and at the end of the reading gradient magnetic field Grat sequentially shifted levels as indicated in FIG. 13 according to thehigh speed spin echo scheme.

Then, the data acquisition is carried out every time the readinggradient magnetic field Gr is switched, and this pulse sequence isrepeated For as many times as necessary at the repetition time TR, suchthat the T₂ images can be obtained From the data acquired by the dataacquisitions 1-1, 1-2, 1-3, etc., while the T₂ * images can be obtainedfrom the data acquired by the data acquisitions 2-1, 2-2, 2-8, etc. andthe acquisitions 3-1, 3-2, etc. Then, the image processing similar tothat in the first and second embodiment described above can be appliedto the obtained T₂ and T₂ * images.

Next, FIG. 14 shows the pulse sequence of the fourth embodiment of theimaging scheme for imaging the physiological function information in thebody to be examined.

In this fourth embodiment, instead of the echo planar scheme used in thefirst and second embodiments described above, the spin echo scheme isused. Thus, in this case, the 90° RF pulse and the slicing gradientmagnetic field Gs are applied to excite the desired slice region togenerate the free induction decay (FID) NMR signals first. Then, the180° RF pulses and the slicing gradient magnetic field Gs are applied,and the reading gradient magnetic field Gr is repeatedly switched topositive and negative alternately to generate a plurality of echosignals, while the phase encoding gradient magnetic field Ge is appliedbefore the reading gradient magnetic field Gr at sequentially asindicated in FIG. 14 according to the spin echo scheme.

Then, the data acquisition is carried out every time the readinggradient magnetic field Gr is switched, and this pulse sequence isrepeated for as many times as necessary at the repetition time TR, suchthat the T₂ images can be obtained from the data acquired by the dataacquisitions 1, while the T₂ * images can be obtained from the dataacquired by the data acquisitions 2, and 3.

Next, FIG. 15 and FIG. 18 show the pulse sequences of the fifth andsixth embodiments of the imaging scheme for imaging the physiologicalfunction information in the body to be examined.

In these fifth and sixth embodiments, the first and third embodiments ofFIG. 4 and FIG. 18 described above are adapted to the three dimensionalimaging scheme. In either one or these two cases, the phase encoding isalso made by the slicing gradient magnetic field Gs and the pulsesequence is repeatedly executed to obtain the three dimensional data.For these three dimensional data, the image processing similar to thatdescribed above can be expanded into three dimensions in an obviousmanner.

It is also possible to apply the imaging scheme for imaging thephysiological function information in the body to be examined accordingto the present invention to the other known pulse sequence such as thatof the gradient echo scheme.

In a case there is a directionality in the body movements of the body tobe examined, it is possible to set up the imaging direction such thatthe influence due to the body movements can be suppressed, and it isalso possible to utilize the ECG gated imaging, or the application ofthe rephasing gradient magnetic Field pulses for suppressing theinfluence of the body movements, or the scheme for suppressing thesignals from the cerebrospinal fluid in a case of imaging the headportion.

As described, according to the present invention, it is possible torealize the nuclear magnetic resonance imaging of the living bodyinformation such as the blood flow change or the oxygen density in bloodrelated to the physiological function change in the living body,quantitatively, at high precision, without being influenced by the bodymovements of the body to be examined, the transverse relaxation time ofthe target nuclear spins, the static magnetic field inhomogeneity, andthe image signal strength change, so that the living body informationhighly useful in the investigation of the living body function and thediagnosis of diseases can be obtained non-invasively.

It is to be noted here that, besides those already mentioned above, manymodifications and variations of the above embodiments may be madewithout departing from the novel and advantageous features of thepresent invention. Accordingly, all such modifications and variationsare intended to be included within the scope of the appended claims.

What is claimed is:
 1. A method of nuclear magnetic resonance imaging,comprising the steps of:imaging a body to be examined placed in ahomogeneous static magnetic field by applying radio frequency magneticfield and gradient magnetic fields according to a pulse sequence,detecting nuclear magnetic resonance signals emitted from the body to beexamined in response to the radio frequency magnetic field and thegradient magnetic fields, and processing the nuclear magnetic resonancesignal to construct nuclear magnetic resonance images; controlling thepulse sequence to realize a first imaging scheme sensitive to afunctional property of the body to be examined, for obtaining first typenuclear magnetic resonance images which are easily affected by amicroscopic magnetic field inhomogeneity, and a second imaging schemeinsensitive to the functional property of the body to be examined, forobtaining second type nuclear magnetic resonance images which are hardlyaffected by the microscopic magnetic field inhomogeneity, so as toobtain the first and second types of the nuclear magnetic resonanceimages corresponding to the first and second imaging schemes,respectively, by a single execution of the imaging step; and obtainingfunctional information of the body to be examined by processing thefirst and second types of the nuclear magnetic resonance images.
 2. Themethod of claim 1, wherein the first imaging scheme is an imaging schemein which the nuclear magnetic resonance signals are detected in a statein which phases of target nuclear spins are disturbed by a microscopicmagnetic field inhomogeneity, while the second imaging scheme is animaging scheme in which the nuclear magnetic resonance signals aredetected in a state in which the phases of the target nuclear spins arenot disturbed.
 3. The method of claim 2, wherein at the controllingstep, the pulse sequence is controlled such that the second imagingscheme is carried out before the phases of the target nuclear spins aredisturbed by a microscopic magnetic field inhomogeneity.
 4. The methodof claim 2, wherein at the controlling step, the pulse sequence iscontrolled such that the second imaging scheme is carried out after thephases of the target nuclear spins are re-aligned.
 5. The method ofclaim 1, wherein the first type nuclear magnetic resonance images areT₂ * images while the second type nuclear magnetic resonance images areT₂ images.
 6. The method of claim 1, wherein at the obtaining step, thefunctional information indicates a living body information related to aphysiological function of the body to be examined.
 7. The method ofclaim 1, wherein at the obtaining step, the functional informationindicates a change of at least one of an oxygen density in blood and alocal blood flow in the body to be examined.
 8. The method of claim 1,wherein at the controlling step, the pulse sequence is controlled to becarried out in at least one of states before, while, and after astimulation is given to the body to be examined.
 9. The method of claim1, further comprising steps of:calculating image correction valuesconcerning a static magnetic field inhomogeneity correction according tothe first and second types of the nuclear magnetic resonance images; andcorrecting the first and second types of the nuclear magnetic resonanceimages by the image correction values; wherein at the obtaining steps,the functional information is obtained by processing the first andsecond types of the nuclear magnetic resonance images corrected at thecorrecting step.
 10. The method of claim 1, wherein at the controllingstep, the pulse sequence is controlled to be carried out at least twice,with and without a stimulation given to the body to be examined, and atthe obtaining step, the functional information is obtained from thefirst type nuclear magnetic resonance images corresponding to the firstimaging scheme carried out with and without the stimulation.
 11. Themethod of claim 10, further comprising steps of:calculating imagecorrection values concerning at least one of a geometrical correctionand a signal strength correction from the second type nuclear magneticresonance images corresponding to the second imaging scheme carried outwith and without the stimulation; and correcting the first and secondtypes of the nuclear magnetic resonance images by the image correctionvalues; wherein at the obtaining steps, the functional information isobtained by processing the first and second types of the nuclearmagnetic resonance images corrected at the correcting step.
 12. Themethod of claim 10, wherein the obtaining step further includes thesteps of:normalizing the first type nuclear magnetic resonance imagesaccording to a picture element value distribution in the second typenuclear magnetic resonance images; and quantifying a change of thefunctional information of the body to be examined from the first typenuclear magnetic resonance images corresponding to the first imagingscheme carried out with and without the stimulation, which arenormalized at the normalizing step.
 13. The method of claim 10, whereinthe obtaining step further includes the steps of:correcting the firsttype nuclear magnetic resonance images by a phase correction concerninga homogeneity of the static magnetic field; and quantifying a change ofthe functional information of the body to be examined from the firsttype nuclear magnetic resonance images corresponding to the firstimaging scheme carried out with and without the stimulation, which arecorrected at the correcting step.
 14. The method of claim 1, wherein atthe imaging step, the pulse sequence for the first imaging schemeincludes:an application of a 90° RF pulse and a slicing gradientmagnetic field to excite a desired slice region to generate freeinduction decay nuclear magnetic resonance signals; and an execution ofa first data acquisition by applying a reading gradient magnetic fieldwhich is repeatedly switched to positive and negative alternately togenerate a plurality of echo signals, while applying a phase encodinggradient magnetic field at a timing of each echo signal; while the pulsesequence for the second imaging scheme includes:an application of a 180°RF pulse to generate the spin echo signals; and an execution of a seconddata acquisition by applying the reading gradient magnetic field whichis repeatedly switched to positive and negative alternately to generatea plurality of echo signals, while applying the phase encoding gradientmagnetic field at a timing of each echo signal.
 15. The method of claim1, wherein at the imaging step, the pulse sequence for the first imagingscheme includes:an application of a 90° RF pulse and a slicing gradientmagnetic field to excite a desired slice region to generate freeinduction decay nuclear magnetic resonance signals; and an execution ofa First data acquisition by applying a reading gradient magnetic fieldwhich is repeatedly switched to positive and negative alternately togenerate a plurality of echo signals, while applying a phase encodinggradient magnetic field at a timing of each echo signal; while the pulsesequence of the second imaging scheme includes:an application of a 180°RF pulse to generate the spin echo signals; and execution of a seconddata acquisition by applying the reading gradient magnetic field whichis repeatedly switched to positive and negative alternately to generatea plurality of echo signals, while applying the phase encoding gradientmagnetic field at a timing of each echo signal; and an execution ofsubsequent data acquisitions by repeating an application of the 180° RFpulse and an application of the reading gradient magnetic field and thephase encoding gradient magnetic field.
 16. The method of claim 1,wherein the pulse sequence for the first and second imaging schemesincludes:an application of a 90° RF pulse and a slicing gradientmagnetic field to excite a desired slice region to generate freeinduction decay nuclear magnetic resonance signals; and an execution ofa plurality of data acquisitions by repeatedly applying a 180° RF pulsesand the slicing gradient magnetic field, while applying a readinggradient magnetic field which is repeatedly switched to positive andnegative alternately to generate a plurality of echo signals and a phaseencoding gradient magnetic field before and at an end of the readinggradient magnetic field at sequentially shifted levels during a periodbetween successive applications of the 180° RF pulse and the slicinggradient magnetic field.
 17. The method of claim 1, wherein the pulsesequence for the first and second imaging schemes includes:anapplication of a 90° RF pulse and a slicing gradient magnetic field toexcite a desired slice region to generate free induction decay nuclearmagnetic resonance signals; and an execution of a plurality of dataacquisitions by applying a 180° RF pulses and the slicing gradientmagnetic field, while applying a reading gradient magnetic field whichis repeatedly switched to positive and negative alternately to generatea plurality of echo signals and a phase encoding gradient magnetic fieldbefore and at an end of the reading gradient magnetic field atsequentially shifted levels.
 18. An apparatus for nuclear magneticresonance imaging, comprising:imaging means for imaging a body to beexamined placed in a homogeneous static magnetic field by applying radiofrequency magnetic field and gradient magnetic fields according to apulse sequence, detecting nuclear magnetic resonance signals emittedfrom the body to be examined in response to the radio frequency magneticfield and the gradient magnetic fields, and processing the nuclearmagnetic resonance signals to construct nuclear magnetic resonanceimages; control means for controlling the pulse sequence to realize afirst imaging scheme sensitive to a functional property of the body tobe examined, for obtaining first type nuclear magnetic resonance imageswhich are easily affected by a microscopic magnetic field inhomogeneity,and a second imaging scheme insensitive to the functional property ofthe body to be examined, for obtaining second type nuclear magneticresonance images which are hardly affected by the microscopic magneticfield inhomogeneity, so as to obtain the first and second types of thenuclear magnetic resonance images corresponding to the first and secondimaging schemes, respectively, by a single execution of the pulsesequence; and processing means for obtaining functional information ofthe body to be examined by processing the first and second types of thenuclear magnetic resonance images.
 19. The apparatus of claim 18,wherein the first imaging scheme is an imaging scheme in which thenuclear magnetic resonance signals are detected in a state in whichphases of target nuclear spins are disturbed by a microscopic magneticfield inhomogeneity, while the second imaging scheme is an imagingscheme in which the nuclear magnetic resonance signals are detected in astate in which the phases of the target nuclear spins are not disturbed.20. The apparatus of claim 19, wherein the control means controls thepulse sequence such that the second imaging scheme is carried out beforethe phases of the target nuclear spins are disturbed by the microscopicmagnetic field inhomogeneity.
 21. The apparatus of claim 19, wherein thecontrol means controls the pulse sequence such that the second imagingscheme is carried out after the phases of the target nuclear spins arere-aligned.
 22. The apparatus of claim 18, wherein the first typenuclear magnetic resonance images are T₂ * images while the second typenuclear magnetic resonance images are T₂ images.
 23. The apparatus ofclaim 18, wherein the processing means obtains the functionalinformation indicating a living body information related to aphysiological function of the body to be examined.
 24. The apparatus ofclaim 18, wherein the processing means obtains the functionalinformation indicating a change of at least one of an oxygen density inblood and a local blood flow in the body to be examined.
 25. Theapparatus of claim 18, wherein the control means controls the imagingmeans to carry out the pulse sequence in at least one of states before,while, and after a stimulation is given to the body to be examined. 26.The apparatus of claim 18, further comprising:means for calculatingimage correction values concerning a static magnetic field inhomogeneitycorrection according to the first and second types of the nuclearmagnetic resonance images; and means for correcting the first and secondtypes of the nuclear magnetic resonance images by the image correctionvalues; wherein the processing means obtains the functional informationby processing the first and second types of the nuclear magneticresonance images corrected by the correcting means.
 27. The apparatus ofclaim 18, further comprising stimulation means for giving a stimulationto the body to be examined, and wherein the control means controls theimaging means to carry out the pulse sequence at least twice, with andwithout a stimulation given to the body to be examined by thestimulation means, and the processing means obtains the functionalinformation by processing the first type nuclear magnetic resonanceimages corresponding to the first imaging scheme carried out with andwithout the stimulation.
 28. The apparatus of claim 27, furthercomprising:means for calculating image correction values concerning atleast one of a geometrical correction and a signal strength correctionfrom the second type nuclear magnetic resonance images corresponding tothe second imaging scheme carried out with and without the stimulation;and means for correcting the first and second types of the nuclearmagnetic resonance images by the image correction values; wherein theprocessing means obtains the functional information by processing thefirst and second types of the nuclear magnetic resonance imagescorrected by the correcting means.
 29. The apparatus of claim 27,wherein the processing means further includes:means for normalizing thefirst type nuclear magnetic resonance images according to a pictureelement value distribution in the second type nuclear magnetic resonanceimages; and means for quantifying a change of the functional informationof the body to be examined from the first type nuclear magneticresonance images corresponding to the first imaging scheme carried outwith and without the stimulation, which are normalized by thenormalizing means.
 30. The apparatus of claim 27, wherein the processingmeans further includes:means for correcting the first type nuclearmagnetic resonance images by a phase correction concerning a staticmagnetic field inhomogeneity; and means for quantifying a change of thefunctional information of the body to be examined from the first typenuclear magnetic resonance images corresponding to the First imagingscheme carried out with and without the stimulation, which are correctedby the correcting means.
 31. The apparatus of claim 18, wherein theimaging means carries out the pulse sequence in which the pulse sequenceFor the first imaging scheme includes:an application of a 90° RF pulseand a slicing gradient magnetic field to excite a desired slice regionto generate free induction decay nuclear magnetic resonance signals; andan execution of a first data acquisition by applying a reading gradientmagnetic field which is repeatedly switched to positive and negativealternately to generate a plurality of echo signals, while applying aphase encoding gradient magnetic field at a timing of each echo signal;while the pulse sequence for the second imaging scheme includes:anapplication of a 180° RF pulse to generate the spin echo signals; and anexecution of a second data acquisition by applying the reading gradientmagnetic field which is repeatedly switched to positive and negativealternately to generate a plurality of echo signals, while applying thephase encoding gradient magnetic field at a timing of each echo signal.32. The apparatus of claim 18, wherein the imaging means carried out thepulse sequence in which the pulse sequence for the first imaging schemeincludes:an application of a 90° RF pulse and a slicing gradientmagnetic field to excite a desired slice region to generate freeinduction decay nuclear magnetic resonance signals; and an execution ofa first data acquisition by applying a reading gradient magnetic fieldwhich is repeatedly switched to positive and negative alternately togenerate a plurality of echo signals, while applying a phase encodinggradient magnetic field at a timing of each echo signal; while the pulsesequence For the second imaging scheme includes:an application of a 180°RF pulse to generate the spin echo signals; an execution a second dataacquisition by applying the reading gradient magnetic field which isrepeatedly switched to positive and negative alternately to generate aplurality of echo signals, while applying the phase encoding gradientmagnetic field at a timing of each echo signal; and execution ofsubsequent data acquisitions by repeating an application of the 180° RFpulse and an application of the reading gradient magnetic field and thephase encoding gradient magnetic field.
 33. The apparatus of claim 18,wherein the imaging means carries out the pulse sequence in which thepulse sequence For the first and second imaging schemes includes:anapplication of a 90° RF pulse and a slicing gradient magnetic field toexcite a desired slice region to generate free induction decay nuclearmagnetic resonance signals; and an execution of a plurality of dataacquisitions by repeatedly applying a 180° RF pulses and the slicinggradient magnetic field, while applying a reading gradient magneticfield which is repeatedly switched to positive and negative alternatelyto generate a plurality of echo signals and a phase encoding gradientmagnetic field before and at an end of the reading gradient magneticfield at sequentially shifted levels during a period between successiveapplications of the 180° RF pulse and the slicing gradient magneticfield.
 34. The apparatus of claim 18, wherein the imaging means carriesout the pulse sequence in which the pulse sequence for the first andsecond imaging schemes includes:an application of a 90° RF pulse and aslicing gradient magnetic field to excite a desired slice region togenerate free induction decay nuclear magnetic resonance signals; and anexecution of a plurality of data acquisitions by applying a 180° RFpulses and the slicing gradient magnetic field, while applying a readinggradient magnetic field, which is repeatedly switched to positive andnegative alternately to generate a plurality of echo signals and a phaseencoding gradient magnetic field before and at an end of the readinggradient magnetic field at sequentially shifted levels.